Such piezoelectric sensors are non-contact type detection sensors which can be used as detectors of automatic doors, etc., and which use ultrasound as detection media. More specifically, a piezoelectric sensor includes a piezoelectric body that mutually converts mechanical energy into electric energy and vice versa. Such functions are so-called a piezoelectric effect and an inverse piezoelectric effect. For example, when a voltage is applied, a piezoelectric body expands and contracts.
JP55-51568 A discloses a structure of a transducer with a combination of a piezoelectric body and a vibration plate. More specifically, a transducer including a piezoelectric body and a vibration plate is disposed on a mount that holds terminals. Electrical continuity is given between the transducer and the terminals by conductive wires. When a voltage is applied to the piezoelectric body through the terminals and the conductive wires, the transducer is inflected together with expansion and contraction of the piezoelectric body, and such an inflection motion generates mechanical vibration (which is referred to as a resonance phenomenon). Hence, the piezoelectric sensor becomes able to transmit ultrasound.
The transmitted ultrasound is reflected by an object, and when the piezoelectric sensor receives the reflected ultrasound, the transducer is inflected. The transducer obtains a voltage upon generation of a piezoelectric effect in response to the inflection. Hence, the piezoelectric sensor is capable of detecting presence/absence of an object approaching a door and a distance to the object, and thus the control unit of an automatic door can output a drive signal to a motor for opening/closing the door.
Meanwhile, according to the transducer of the prior art, the node of such mechanical vibration is supported by the mount, and the whole nodes of such mechanical vibration contact the mount. More specifically, the mount is provided with a cylindrical rib protruding toward the transducer, and the tip surface of this annularly closed rib supports the transducer. Further, a cylindrical bonding part is formed at the inner circumference of the rib, and the transducer is fixed by a bond at the nodes of vibration.
According to such a structure, however, unnecessary vibration so-called spurious vibration is generated to the transducer. That is, even if the mount contacts the transducer through nodes where amplitude becomes zero, when the contact area between the mount and the transducer is large, the mount disturbs vibration of the transducer. As a result, spurious vibration is generated and deteriorates the vibration performance of the transducer.
According to the structure that simply supports the transducer by the cylindrical rib and the bonding parts, no special consideration is given to the generation of spurious vibration, and any measures for reducing the spurious vibration become necessary. Moreover, there is a technique which fixes the mount to the transducer by a bond in a non-contact manner. However this needs a jig, etc., for suspending the transducer, thereby making the production of the piezoelectric sensor difficult. Hence, it is an object of the present invention to provide a piezoelectric sensor which can address the above-explained problem and which can reduce the spurious vibration of a transducer.